This application claims priority to U.K. application number 9823199.6 filed on Oct. 24, 1998.
1. Field of the Invention
The present invention is concerned with the provision of a switch to control the de-adjustment of a vehicle brake when a service operation is required, such as to replace worn pads.
2. Description of the Related Art
It is already well known to provide an electric motor to control the running clearance of braking linings relative to a brake rotor based upon signals from sensors monitoring the clearance take-up movement and brake actuation stroke. The known systems tend to mimic the mechanical operation of a conventional brake clearance control device known as an xe2x80x9cautomatic adjusterxe2x80x9d. In such adjusters, a clutch having some degree of lost motion is provided where the level of free motion is equivalent to the maximum allowable running clearance. Should the brake linings wear such that the running clearance is greater than this maximum allowable clearance, upon operation of the brake, the free running clearance is xe2x80x9ctaken-upxe2x80x9d and the further additional movement, caused by the excess lining clearance causes the clutch to slip. The slipping motion has the effect of moving the backstop or datum position for the return of the brake lining, thus progressively advancing the lining towards the brake rotor as the brake lining wears. Such mechanical automatic adjusters are well known in the art.
In a brake of the kind with which the switch of the present invention is concerned, it is important to reduce weight, power consumption (whether electrical or pneumatic) and material costs. In a conventional brake, having an automatic adjuster of the mechanical kind or even an electric adjuster that mimics the mechanical operation, the strength of the mechanisms associated with the adjuster has to be extremely high. The reason for this is that the actual brake adjustment takes place only during the brakes-on phase of a brake application. The adjuster mechanism moves only when brake torque is being applied and therefore the adjuster drive train has to be capable of driving through this locked in torque.
We have proposed in another Application filed concurrently herewith that in a brake adapted to apply a brake lining to a brake rotor, there is provided a clearance control device for actively maintaining the free running clearance between the brake lining and the brake rotor during the non-braking phase, wherein the adjustment of the brake lining position relative to the brake rotor only takes place during the brakes-off condition.
The advantage of only adjusting the position of the brake lining in the brakes-off condition is that the locked-in brake torque that has to be overcome before adjustment of the conventional kind can take place has now been eliminated.
Therefore in such a brake the torque required to move the brake linings into the new datum position, when the brakes are not applied, is substantially reduced, and likewise, the torque being carried by the adjustment mechanism is also reduced.
With the reduction in torque required to perform the adjustment, it is now possible to redesign the adjuster drive train so that it is substantially smaller and lighter without compromising the performance of the adjustment mechanism.
In such a situation, it is now possible to design and implement an adjuster mechanism that is substantially formed from a light weight plastics material, which enables the overall weight of the brake assembly to be drastically reduced.
The electric motor can be housed within the brake, thus protecting it and it""s associated drive from any ingress of contaminants.
The invention is particularly applicable to brakes of the type wherein angular displacement of a rotary member, referred to commonly as the op-shaft, by an external actuator causes axial displacement of a pair of laterally spaced tappets, by way of rollers running on cam surfaces of the rotary member, the displacement of the tappets transmitting the actuator input load to a brake lining of the brake.
The electric motor can be located in the portion of the brake between the tappets, in a portion of the brake that conventionally was occupied by a mechanical adjuster. In such a position, the motor and it""s drive are in close association with the mechanism for applying the brake.
In order to reduce the electric power consumption to a low level, the electric motor employed can be a high-speed low power/torque motor, operated in use in combination with a reduction gearbox.
The reduction gearbox can be of a compact design such as a compound epicyclical or multi-stage planetary gearbox. Alternative designs such as worm and wheel or cycloidal or any combination thereof are also suitable.
In order to provide the adjustment data required to perform the adjustment operation, a sensor or sensors is/are provided to generate information about the movement of the brake. In particular, the free running stroke of the brake, i.e. the clearance take-up movement, needs to be assessed. The stroke of the brake actuator can be measured by a displacement sensor, but the portion of that travel that is equivalent to the free running travel can only be determined by assessment of the displacement in combination with knowing the load being applied. As the brake lining moves, under normal actuation, the load being applied during the free running or clearance take-up movement is low. As soon as the brake linings engage the brake rotor, the load required to move any distance further is substantially increased. It can therefore be seen that by comparison of brake lining displacement and applied load, the free running clearance of the brake lining with respect to the brake rotor can be determined.
Therefore, a sensing means can be provided which is adapted to determine both the displacement of the brake and the load applied by the brake and a means to compare the output of the sensing means so as to determine the free running clearance of the brake.
The free running clearance of the brake can then be maintained within a desired range, through use of an electric drive within the brake adapted to adjust the datum position of the brake linings.
A sensor within the brake provides brake lining position information that can be used to control the motor in positioning the brake linings in correct accordance with the prescribed limits.
The sensor can also be used to provide a signal that indicates the actual wear state of the brake linings and may take the form of an encoder, either linear or rotary, that is driven from a part of the brake that moves upon adjustment of the brake, said encoder signals being accumulated in a non-volatile memory for use in an adjustment operation.
In a brake having an electric motor adapted to perform the adjustment operation, de-adjustment can also be enabled automatically through use of the same motor. De-adjustment is required when it has been determined that the brake linings require replacement. In a conventional brake, the adjuster mechanism has to be manually unwound so as to retract the brake applying piston away from the brake rotor. This enables the old linings to be removed and replaced with new linings. When an electric motor is used, a simple electrical signal from a brake associated switch, either mounted on the brake or electrically connected thereto, can be used to instigate the retraction of the brake applying member. It is with such a switch that the present invention is concerned.
In accordance with the present invention in its broadest aspect, there is provided a switch to supply a de-adjust signal to a brake of the kind having electrically actuated adjustment, the switch being adapted to be mounted on or in the body of a caliper of the brake and to provide a non-contact switch arrangement actuated by a rotary movement provided by a rotatable component of the brake in response to lining wear.
In accordance with a second aspect of the invention, there is provided a vehicle brake of the caliper type having electrically actuated adjustment of the running clearance of a brake lining relative to a brake rotor, wherein, for brake operation, angular displacement of a rotatable component of the brake by an external actuator causes axial displacement of a tappet arrangement within the brake whereby to transmit the actuator input load to the brake lining of the brake, and wherein a switch is provided to supply a brake de-adjust signal, characterised in that the switch is adapted to be mounted on or in the body of the brake caliper and to provide a non-contact switch arrangement actuated by a rotary movement provided by said rotatable component of the brake in response to lining wear.
Preferably, the switch comprises two relatively rotatable components which carry respective parts of a two-part, non-contact type switch arrangement.
In one advantageous embodiment, one component carries a magnet and the other carries a Hall-effect sensor.
In a preferred embodiment, the two relatively rotatable components comprise a base frame, carrying the Hall-effect sensor, and an upper cover carrying the magnet, the frame and cover being assembled through a boss on or coupled to the caliper housing such that the cover can be selectively rotated about the frame.
In a particularly preferred embodiment, the cover turns on the frame such as to provide both rotational and linear movement therebetween. This can be achieved by providing a lead screw between these components.
Advantageously, the magnet and sensor are both disposed off-axis on the relatively rotatable components, with the magnet being substantially 180 degrees opposed to the sensor in a rest/start position and wherein relative rotary movement by an amount at which a signal is to be generated brings the magnet and sensor into alignment.